FIG. 8 is a block diagram illustrating a structure of a background image forming apparatus having a so-called finisher function. Generally, the finisher function refers to a function to staple and/or punch sheets output from an image forming apparatus. As shown in FIG. 8, an image forming apparatus 500 is constructed of an image forming unit 501 and a finisher 2. A control unit 511 of the image forming unit 501 controls an operation of the image forming apparatus 500. When a print command is received from an external host computer "C", an image generating unit 12 interprets the print command and generates bit-mapped image data, and an image forming device 13 forms an image on a sheet according to the image data. The finisher 2 includes a sheet output tray 21 on which sheets are output from the image forming unit 501, a staple unit 22 which staples the output sheets, and a punch unit 23 which punches the output sheets. The finisher 2 staples or punches the output sheets according to a staple or punch instruction included in the print command. When the instruction for stapling or punching is input through an operational display panel 14, the control unit 511 controls the finisher 2 to staple or punch the output sheets according to the input instruction.
FIG. 9 is a schematic drawing illustrating a construction of the finisher 2 shown in FIG. 8. Numeral 20 denotes a staple/punch unit (a combination of 22, 23 from FIG. 8). The staple/punch unit 20 is movable along an edge of the sheet output tray 21. The unit 20 is also movable along a curved line at the side of an user "U" operating the apparatus 500. A letter "M" denotes an area where the staple/punch unit 20 is movable.
FIG. 10 is a schematic drawing illustrating positions where the staple/punch unit 20 can staple or punch. As illustrated in FIG. 10, the staple/punch unit 20 can staple or punch sheets in a position "S" or "H" in parallel with a side P1 of a sheet "P"(in parallel with the right side edge of the sheet "P" when viewed from the user "U") or staples slantingly in a position "S" at a side corner P2 of the sheet "P" (at the front right side corner of the sheet "P" when viewed from the user "U").
FIG. 11 is a diagram illustrating an example of a menu for selecting a stapling/punching position and a stapling/punching pattern, which is displayed in the operational display panel 14 of the apparatus 500 or a display of the host computer "C". As menus for specifying a stapling position, there are shown such positions as "right side", "left side", "upper right", "upper left", "lower right"and "lower left" positions. As menus for a stapling pattern, there are shown such patterns as "horizontal", "perpendicular", "slanting" and "double" patterns. The user "U" selects and inputs a desired staple position and a desired staple pattern from among these menu choices. A punching position and a punching pattern are also selected and input in a similar manner. Also, there are shown menus for selecting a duplex print (forming an image on both sides of a sheet) or a simplex print (forming an image on one side of a sheet), and menus for selecting a binding side for the duplex print. When the user selects the duplex print and specifies a binding side for the sheet, such as "upper" binding, "left" binding or "right" binding, the apparatus forms an image on a sheet so as to have a binding margin at the specified side.
FIGS. 12(a) and 12(b) are diagrams illustrating conceivable positions and patterns for stapling and punching, respectively. It is possible to staple or punch sheets in all of the positions shown in FIGS. 12(a) and 12(b) if the staple/punch unit 20 can be moved along and around the entire circumference of the sheet "P". However, the finisher 2 will become complicated and large and consequently expensive if the staple/punch unit 20 is so constructed as to move around the entire circumference of the sheet "P". Therefore, the staple/punch unit 20 is generally so constructed as to move only in a predetermined area as illustrated in FIG. 9.
Accordingly, the image forming apparatus 500 can staple or punch only in the positions shown in FIG. 10. In other words, the image forming apparatus 500 can not staple or punch in other positions than those shown in FIG. 10.
When the image forming apparatus 500 is used in a network environment, the apparatus 500 is rarely placed at a location near the user "U". Therefore, generally the user "U" inputs the position and the pattern of stapling or punching without knowing the feeding direction of the sheets in the apparatus 500. For example, when the user inputs to staple in the "left side" position with "double" pattern for a portrait image and when the sheet "P" in the apparatus 500 is fed sideways as illustrated in FIG. 13(a), the apparatus 500 can staple the sheets as input because the "left side" of the sheet "P" is within the area "M". However, when the sheet "P" is fed lengthwise as shown in FIG. 13(b), then the apparatus 500 cannot staple in the "left side" position as input because the "left side" of the sheet "P" is not within the area "M". In such a case, generally the print command is canceled as an error and the printing operation is not performed. Consequently, printing productivity is lowered. A similar problem occurs when punching.